In general, a seat belt apparatus and an air bag apparatus are widespread in view of protection of a rider of an automobile. The seat belt apparatus is equipped in order to prevent the rider from being thrown from his/her seat in the automobile due to an impact resulting from a collision of the vehicle, and binds/fastens the rider to the seat by lapping a belt around the body of the rider.
In recent years, a seat belt apparatus having a pretensioner function for improving a rider protecting function has rapidly become widespread. According to this pretensioner function, the seat belt loosened due to the thickness of clothes or the like is instantaneously wound up upon a collision or immediately before the collision, in order to improve an effect of binding the rider. This pretensioner function is implemented by strongly winding up the seat belt with a gas pressure output from a gas generator referred to as a micro gas generator.
In relation to a seat belt windup drive unit provided with this gas generator, various mechanisms are known. A seat belt windup drive unit formed by annularly coiling a metal belt coupled to a shaft winding up a seat belt and holding the metal belt with a pair of platelike members to block a hollow part of the annularly coiled metal belt while arranging a gas generator in a space defined by the metal belt and the pair of platelike members is known as one of such mechanisms (refer to specification of German Patent Publication No. 19961109 (Patent Document 1)).
In the seat belt windup drive unit disclosed in the aforementioned Patent Document 1, the pressure in the space defined by the metal belt and the pair of platelike members rises due to gas output from the gas generator to expand the metal belt outward, thereby rotating the shaft by a prescribed quantity and winding up the seat belt.
Patent Document 1: Specification of German Patent Publication No. 19961109